A scanning electron microscope (SEM) used for observation, inspection and measurement of a sample using an electron beam accelerates electrons emitted from an electron source and irradiates the electrons so as to be converged onto the sample surface by an electrostatic or electromagnetic lens. They are called primary electrons. By incidence of the primary electrons, secondary electrons and reflective electrons are generated from the sample. By detecting these secondary electrons and reflective electrons while deflecting and scanning an electron beam, a scanning image of a fine pattern and a composition distribution on the sample can be acquired.
As problems in the scanning electron microscope, high speed and high accuracy correction of a focus error and astigmatism can be cited.
The most common method as the measurement method and correction method for astigmatism is a method for obtaining the astigmatism from the qualities of two-dimensional picture images acquired while changing the intensity of the astigmatism corrector. As an index of the picture quality, contrast of a picture image, sharpness of a contour of a pattern and the like are used, and measurement and correction of the astigmatism are executed by maximizing these indexes.
On the other hand, to use a method of measuring the astigmatism from the change in the position displacement of an electron beam which occurs when the electron beam is tilted has been proposed in Japanese Unexamined Patent Application Publication No. 2007-141866, Japanese Unexamined Patent Application Publication No. 2001-357811, Japanese Unexamined Patent Application Publication No. 2004-146192, Japanese Unexamined Patent Application Publication No. H07-262952 and the like. In Japanese Unexamined Patent Application Publication No. 2007-141866, it is disclosed that an electric current value of an astigmatism coil required for making the astigmatism quantity 0 is calculated based on the result of an astigmatism analysis by parallax, and an astigmatism correction coil is conditioned. In Japanese Unexamined Patent Application Publication No. 2001-357811, it is disclosed that the astigmatism is corrected using an astigmatism corrector based on information of an elliptical figure formed from an image displacement quantity by tilting. In Japanese Unexamined Patent Application Publication No. 2004-146192, it is disclosed that the astigmatism is corrected from the relation between the astigmatism quantity and the strength of an astigmatism correction coil that has been known beforehand from an image displacement quantity by tilting. In Japanese Unexamined Patent Application Publication No. H07-262952, it is disclosed that excitation control of an astigmatism correction coil is executed while observing image displacement by tilting.